Proton (¹H) NMR spectroscopy

· Proton NMR is used to analyse hydrogen environments in simple organic molecules.
· A ¹H NMR spectrum gives information about: number of proton environments, types of proton environments, relative number of protons, and neighbouring protons.
· Use ¹H NMR with other analytical data, especially molecular formula, IR, mass spectrometry, and carbon-13 NMR, to deduce a structure.
· In exams, always combine chemical shift, integration, and splitting pattern before deciding the final structure.

Chemical shift: proton environments

· Chemical shift, δ = position of a proton signal, measured in ppm.
· Different chemical environments give different chemical shift values.
· Protons near electronegative atoms or π bonds are usually more deshielded and appear at higher δ / further left / downfield.
· Use the data booklet chemical shift table to identify likely proton environments.
· Typical guide: alkyl C–H ≈ 0.5–2 ppm; C–H next to C=O / C=C / aryl ≈ 2–3 ppm; C–H next to O, N or halogen ≈ 3–4 ppm; alkene H ≈ 4.5–6 ppm; aromatic H ≈ 6–8 ppm; aldehyde H ≈ 9–10 ppm; carboxylic acid O–H ≈ 10–12 ppm.
· Equivalent protons are in the same environment and give one signal.
· A molecule with symmetry may have fewer signals than the total number of H atoms suggests.

Peak area / integration

· Peak area or integration shows the relative number of protons in each environment.
· Integration values are a ratio, not always the exact number of protons directly.
· Simplify the integration ratio to the smallest whole-number ratio.
· Match the ratio to the molecular formula to find the actual number of protons in each environment.
· Example: integration ratio 3 : 2 may mean CH₃ : CH₂, but only if consistent with the full structure.
· Do not identify a group from integration alone; combine it with chemical shift and splitting.

This diagram shows that integration measures the relative area under each signal. The labelled examples link the integration ratio to the number of protons in different environments. Source

Splitting patterns and the n + 1 rule

· Splitting pattern shows the number of equivalent protons on adjacent carbon atoms.
· Use the n + 1 rule: if a proton environment has n equivalent neighbouring protons, its signal is split into n + 1 peaks.
· 0 neighbouring H → singlet.
· 1 neighbouring H → doublet.
· 2 neighbouring H → triplet.
· 3 neighbouring H → quartet.
· Many nearby non-equivalent protons may give a multiplet.
· The n + 1 rule is applied to protons on adjacent carbon atoms, not usually to protons on the same carbon.
· Equivalent protons do not split each other.
· Splitting helps identify fragments such as CH₃CH₂–: the CH₃ often appears as a triplet, and the CH₂ often appears as a quartet.

These diagrams show how neighbouring protons split signals into patterns such as doublets, triplets, and quartets. They are useful for practising the n + 1 rule in exam-style structure identification. Source

TMS and the chemical shift standard

· Tetramethylsilane, TMS, is used as the standard for chemical shift measurements.
· TMS is assigned δ = 0 ppm.
· Chemical shifts are measured relative to TMS.
· TMS gives one sharp singlet because all 12 protons are equivalent.
· TMS is suitable because it is chemically inert, gives a strong signal, and usually does not overlap with organic compound proton signals.

This page explains how δ values are measured relative to TMS at 0 ppm. The diagrams support understanding of upfield, downfield, shielding, and deshielding. Source

Deuterated solvents

· ¹H NMR samples are dissolved in deuterated solvents, such as CDCl₃.
· A normal solvent containing many ¹H atoms would produce a large proton signal and interfere with the sample spectrum.
· Deuterium, ²H, is used instead of ¹H so the solvent gives minimal interference in the ¹H NMR spectrum.
· Common exam phrase: deuterated solvents are needed so that solvent protons do not produce peaks in the ¹H NMR spectrum.

D₂O exchange: identifying O–H and N–H protons

· O–H and N–H protons can be identified using proton exchange with D₂O.
· Add D₂O and run the ¹H NMR spectrum again.
· Signals due to O–H or N–H protons disappear or greatly reduce because H is exchanged for D.
· C–H proton signals do not disappear after D₂O exchange.
· This is especially useful for identifying alcohols, phenols, carboxylic acids, amines, and amides.

Deduce a structure from a ¹H NMR spectrum

· Step 1: Count the number of signals → number of different proton environments.
· Step 2: Use chemical shifts → identify likely environments / functional group surroundings.
· Step 3: Use integration ratios → find relative numbers of each type of proton.
· Step 4: Use splitting patterns → find neighbouring equivalent protons using the n + 1 rule.
· Step 5: Assemble possible fragments and check against the molecular formula.
· Step 6: Confirm the structure by checking that every proton environment has the correct shift, area, and splitting.
· Exam tip: if two possible structures fit the formula, choose the one that matches all NMR evidence, not just one peak.

Predict a ¹H NMR spectrum from a molecule

· Identify all sets of equivalent protons.
· Predict the number of signals from the number of proton environments.
· Predict each chemical shift using the proton environment and data booklet values.
· Predict relative peak areas from the number of protons in each environment.
· Predict splitting by counting equivalent protons on adjacent carbon atoms and applying n + 1.
· Remember: O–H and N–H signals are often variable and may appear broad; they can be confirmed using D₂O exchange.